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Industrial Ethernet Networks

2-8

SIMATIC NET Twisted-Pair and Fiber-Optic Networks

C79000-G8976-C125-02

2.4 Switching

Basic Principles of Switching

Switches forward data packets directly from the input port to the output port based

on the address information in the data packet. Switches allow, as it were, a direct

interconnection.

A switch has essentially the following functions:

S Connecting Collision Domains / Subnets

Since repeaters and hubs (star couplers) function at the physical layer, their use

is restricted to the span of a collision domain.

Switches interconnect collision domains. Their use therefore is not restricted to

the maximum span of a repeater network. Switches actually permit very large

networks to be implemented with spans of up to 150 km.

S Load Containment

By filtering the data traffic based on the Ethernet (MAC) addresses, local data

traffic remains local. In contrast to repeaters or hubs, which distribute data

unfiltered to all ports / network nodes, switches operate selectively. Only data

intended for nodes in other subnets is switched from the input port to the

appropriate output port of the switch.

To make this possible, a table assigning Ethernet (MAC) addresses to output

ports is created by the switch in a “teach-in” mode.

S Limitation of Errors to the Network Segment Affected

By checking the validity of a data packet on the basis of the checksum which

each data packet contains, the switch ensures that bad data packets are not

transported further. Collisions in one network segment are not passed on to

other segments.

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